Method and apparatus for compensation for weather-based attenuation in a satellite link

ABSTRACT

A controlled satellite link, comprising a reference unit for measuring signal attenuation over said link, and a control unit, associated with said reference unit, configured for controlling at least one link transmission parameter to dynamically adapt to changes in said measured signal attenuation.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/907,930, filed on Apr. 23, 2007, the contents ofwhich are hereby incorporated by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a system for compensation forweather-based attenuation in a satellite link and, more particularly,but not exclusively to such compensation using one or both of automaticuplink power control and adaptive coding and modulation for satellitecommunications. The present methods can be used in either one direction,that is forward or return links, or in both directions (forward andreturn links).

Weather can cause attenuation to the signal on a satellite communicationlink. Furthermore the ground to satellite leg may experience differentweather conditions from the satellite to ground leg. Furthermore, in abroadcast system, different satellite to ground legs may experiencedifferent weather conditions, so that the overall attenuation in thelink may not only change rapidly but may differ between differentreceiving stations at the same instant.

A number of solutions have been used in the past. One popular solutioninvolves designing the satellite communication system at the outset forthe worst case weather conditions. Such a solution is particularlywasteful of power although it rarely fails. Another solution involvesusing climatology to estimate weather parameters of concern, and thensetting the transmission power for the estimated conditions. Furthersolutions use empirical models based on climatological data and longbaseline observations of signal strength to model RF attenuation andcompensate accordingly.

A recent proposal involves operating the link based on expected dailyweather conditions for the specific geographical region in which thelink operates. However even in this case transmission power is wastedsince the system operates on the basis of the worst case within the timeand geographical frame of the estimate.

It is known to provide automatic uplink power control (AUPC), that is,adjusting the output power on the uplink, with the general aim ofmaintaining a constant signal to noise (SNR) ratio at the remote end.This is however inexact as the control over the downlink is indirect.

Adaptive coding and modulation (ACM) is known to keep the receivedsignal quality (e.g. bit error rate) constant in the face of changingSNR levels. The modulation pattern is changed between a high capacitymodulation at high SNR and a low capacity but highly robust modulationwhen the SNR decreases.

However attempts to dynamically change the modulation based on therequirements of the system as a whole lead to problems with individuallinks. Likewise attempts to dynamically change individual links lead toimbalances in the system as a whole.

The following documents are representative of the state of the art:

Thomas J. Saam, “Uplink Power Control Technique for VSAT Networks”, inProceedings of Southeastcon 89, pp. 96-101, April 89.

Thomas J. Saam, “Uplink power control mechanism for maintaining constantoutput power from satellite transponder”, U.S. Pat. No. 4,941,199, FiledApr. 6, 1989.

Lawrence W. Krebs et al., “Methods and Apparatus For Mitigating RainFading Over Satcom Links Via Information Throughput Adaptation, U.S.Pat. No. 7,174,179, Filed Feb. 6, 2007.

ETSI EN 302 307 V1.1.1 (2004-01): “Digital Video Broadcasting (DVB)Second generation framing structure, channel coding and modulationsystems for Broadcasting, Interactive Services, News Gathering and otherbroadband satellite applications”.

Alberto Morello, Vittoria Mignone, “DVB-S2: The Second GenerationStandard for Satellite Broad-band Services”, Proceedings of the IEEE,vol. 94, no. 1, pp. 210-227, January 2006

G. Maral, M. Bousquet, Satellite Communications Systems, Third Edition,John Wiley & Sons, Ltd., 1999.

SUMMARY OF THE INVENTION

The changing levels of attenuation in the system can be compensated fordynamically by changing the modulation level in the link between the huband the end user. However over the system as a whole rules are set toensure that the hub remains within its resources.

According to one aspect of the present invention there is provided acontrolled satellite link comprising a first link and a second link,comprising:

at least one reference unit for obtaining at least signal attenuationover the controlled satellite link, and

a control unit, associated with the reference unit, configured fordiscriminating between first link attenuation and second linkattenuation and accordingly controlling at least one link transmissionparameter associated with the first link and at least one transmissionparameter associated with the second link to dynamically adapt tochanges in the obtained signal attenuation.

In an embodiment, the controlled transmission parameter is transmissionpower.

In an embodiment, the controlled transmission parameter comprises acoding parameter.

In an embodiment, the controlled transmission parameter comprises amodulation parameter.

In an embodiment, the controlled transmission parameter is transmissionpower in the first link and a coding parameter in the second link.

In an embodiment, the controlled transmission parameter is transmissionpower in the first link and a modulation parameter in the second link.

Additionally or alternatively, the controlled transmission parameter istransmission power in the first link and a combination of coding andmodulation parameters in the second link.

In an embodiment, the second link comprises a plurality of links torespective receiving stations and wherein a plurality of the respectivereceiving stations comprise independent reference units, thereby toallow independent control of the plurality of links.

In an embodiment, the independent reference units are geographicallysubstantially evenly distributed to allow independent control of theplurality of links on a geographical basis.

In an embodiment, measurements by the reference unit are made in aplurality of cycles and the derivation unit is configured to obtain thefirst link attenuation in a first cycle and the second link attenuationin a second cycle.

In an embodiment, the first link is an uplink and the second link is adownlink and wherein the controlled transmission parameter istransmission power in the uplink and a combination of coding andmodulation parameters in the downlink.

According to a second aspect of the present invention there is provideda method of controlling a satellite link, the satellite linktransmitting according to transmission parameters, the methodcomprising:

measuring attenuation over the link, the link having two legs, a firstleg to a satellite and a second leg to respective ones of a plurality ofground based receiver stations, the measuring being carried out at someof the ground based receiver stations, each leg being associated withrespective ones of the transmission parameters, the measuringdiscriminating between attenuation at the respective legs; and

dynamically adjusting at least one of the transmission parametersassociated with the first leg and at least one of the transmissionparameters associated with the second leg to adapt for changes in thediscriminated measured attenuation.

In an embodiment, the at least one transmission parameter adjusted istransmission power and the dynamically adjusting comprises increasingthe transmission power at the first leg to compensate for increases inthe measured attenuation and decreasing the transmission power tocompensate for decreases in the measured attenuation, and adjustingmodulation levels over the second leg such that the error rate afterdecoding of the received signal at the plurality of ground basedreceiver stations is kept substantially constant.

In an embodiment, the at least one transmission parameter adjusted is acoding parameter, and the dynamically adjusting comprises increasingcoding complexity (decreasing code rate) to compensate for increases inthe measured attenuation, and decreasing coding complexity (increasingcode rate) to compensate for decreases in the measured attenuation suchthat the error rate after decoding of the received signal is keptsubstantially constant.

In an embodiment, the at least one transmission parameter adjusted is acoding parameter, and the dynamically adjusting comprises decreasingcode rate to compensate for increases in the measured attenuation, andincreasing code rate to compensate for decreases in the measuredattenuation such that the error rate after decoding of the receivedsignal is kept substantially constant.

In an embodiment, the at least one transmission parameter adjusted is amodulation parameter, and the dynamically adjusting comprises decreasingmodulation constellation size to compensate for increases in themeasured attenuation, and increasing modulation constellation size tocompensate for decreases in the measured attenuation such that the errorrate after decoding of the received signal is kept substantiallyconstant.

In an embodiment, the satellite link comprises a first link and a secondlink, and wherein the dynamically adjusting is carried out independentlyfor the first and second links respectively.

In an embodiment, the at least one parameter is transmission power forthe first link and a coding or modulation parameter for the second link.

In an embodiment, the at least one parameter is transmission power forthe first link and a coding parameter and a modulation parameter for thesecond link.

In an embodiment, the first link is an uplink from a ground-basedtransmitting station to a satellite, and the second link is a downlinkfrom the satellite to a ground-based receiving station.

In an embodiment, the satellite link is a broadcasting link, and thereis a plurality of ground-based receiving stations.

In an embodiment, at least some of the ground-based receiving systemscarry out the measuring, and common attenuation is assigned to theuplink and differential attenuation is assigned to respective downlinks.

According to a third aspect of the present invention there is provided amethod of compensating for dynamic changes in attenuation in a satellitelink having a first link and a second link and attenuation over thefirst link and attenuation over the second link, comprising:

measuring the dynamic changes in attenuation at a receiving station ofthe satellite link,

discriminating using the measurements between dynamic changes relatingto attenuation at the first link and dynamic changes relating to changesat the second link;

feeding the measurements via a return link; and

using the measurements and the discrimination, modifying at least onetransmission parameter associated with the first link and onetransmission parameter associated with the second link to adapt to thedynamic changes.

According to a fourth aspect of the present invention there is provideda satellite receiving station for receiving a channel of a satellitecommunication link; comprising:

a measuring unit for obtaining measurements of dynamic changes inattenuation of the channel, the measurement including data to allowdiscrimination between attenuation in a first leg and attenuation in asecond leg of the satellite link; and

an output unit for sending the measurements via a return channel toallow for channel compensation.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples provided herein are illustrative only and not intended to belimiting.

Implementation of the method and system of the present inventioninvolves performing or completing certain selected tasks or stepsmanually, automatically, or a combination thereof. Moreover, accordingto actual instrumentation and equipment of preferred embodiments of themethod and system of the present invention, several selected steps couldbe implemented by hardware or by software on any operating system of anyfirmware or a combination thereof. For example, as hardware, selectedsteps of the invention could be implemented as a chip or a circuit. Assoftware, selected steps of the invention could be implemented as aplurality of software instructions being executed by a computer usingany suitable operating system. In any case, selected steps of the methodand system of the invention could be described as being performed by adata processor, such as a computing platform for executing a pluralityof instructions.

The changing levels of attenuation in the system can be compensated forby changing the modulation and coding level, the MOD COD in use. Howeverover the system as a whole, with links spread over a geographical areait is desirable to limit the overall number of MOD CODs in use.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin order to provide what is believed to be the most useful and readilyunderstood description of the principles and conceptual aspects of theinvention. The description taken with the drawings makes apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

In the drawings:

FIG. 1 is a simplified diagram illustrating a satellite link dynamicallyadjusted by AUPC & ACM mechanisms to overcome changing weather-basedattenuation according to a first embodiment of the present invention.

FIG. 2 is a simplified diagram showing theoretical measuring of separateuplinks and downlinks for differential control of the links, accordingto a preferred embodiment of the present invention.

FIG. 3 is a simplified diagram showing the control of differentparameters for the uplink and for the downlink according to a preferredembodiment of the present invention.

FIG. 4 is a simplified diagram showing an implementation of the systemof FIG. 3, according to a preferred embodiment of the present invention.

FIG. 5 is a simplified flow chart illustrating a process of changingcontrol parameters for each link in a respective stage followingpolling, according to a preferred embodiment of the present invention.

FIG. 6 is a simplified graph showing resource consumption, that is bothbandwidth and power, for different MODCODS.

FIG. 7 is a simplified flow chart showing different MODCODS withdifferent levels of availability.

FIG. 8 is a graph showing MODCOD distribution in percentages.

FIG. 9 is a schematic illustration showing different MODCODS withdifferent percentage uses for a system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments comprise an apparatus and a method which usesmeasured weather conditions or weather consequential attenuation on thelink to modify the link parameters.

In an embodiment the weather-related attenuation on the uplink, meaningthe link from the originating ground station to the satellite, ismeasured, or more accurately estimated from a measure of the overallattenuation, and the uplink power is controlled accordingly to achieve asubstantially constant received uplink power. In the same embodiment theweather related attenuation on the downlink, meaning the link from thesatellite to the receiving station, is measured, or more accuratelyestimated from the same measurement as before, and the downlinkmodulation and coding parameters are modified to compensate for theattenuation and provide a substantially constant receive quality at thereceiving station.

The present embodiments further modify the transmission on the linkbetween the hub and the home, not by modifying the transmission powersince this is often not possible, but rather by modifying the MODCOD,that is to say modifying the modulation so that at low attenuation (goodweather) high level modulation is used to obtain a high bandwidthchannel. At greater attenuations lower level modulation is used tocompensate for the greater attenuation and still provide correctreception, but at the cost of bandwidth. The hub or part of the hubinvolved in the particular transmission however does not operate usingendless MODCODS for all of its links. Rather, according to the presentembodiments the total number of MODCODs in use at any given time islimited, and the limitation follows a scheme which looks for the bestefficiency from the MODCODs chosen.

In the present embodiments, an ACM controller at the hub sends commandsto the various terminals to control the transmission speed, transmissionpower, and MODCOD for Inbound links. The control is provided with theaim of achieving balanced resource consumption namely balancingBandwidth and Power Equivalent Bandwidth, while taking into account theindividual limitations of the terminals in terms of EIRP, the individualinstantaneous traffic load, and the individual climate conditions at theremote site of each terminal.

The estimation of the climate conditions can be based on the samemeasurements that are used for the Outbound link or separatemeasurements performed by the return links receiver at the Hub. In otherwords, the ACM of the hub at the return link is concerned with balancingof resources. This contrasts with control at the terminal which hasdifferent aims and resources. At the terminal there is no issue ofsharing fixed resources. The terminal's only limitation is transmissionpower to the satellite. At the down link, that is to the earth station,the receiver antenna is so large that attenuation is not really anissue. The only issue for the terminal using the return link is tochange the rate of transmission in accordance with the weather, and usethe highest MODCOD that the weather based attenuation allows.

However from the point of view of the system, bandwidth relates topower, and the power available at the satellite is limited. Thus it isdesirable to use all available power over the system as a whole but nomore. Individual terminals at times of low overall traffic may of coursebe assigned the highest MODCOD. However once power availability does notmeet requirements a lower MODCOD is assigned even if the weather doesnot require it.

Also, for the system as a whole, even in the case of low traffic andgood weather, the satellite may still wish to limit the user of highMODCODs, because of issues with the power equivalent bandwidth. Thepresent embodiments provide a policy for all users, not just those incrisis due to weather etc to assign MODCODs.

More specifically, the present embodiments involve enhancing a VSAT starnetwork based on a single carrier time multiplexed outbound channel(e.g. DVB-S2), with combined AUPC (Automatic Uplink Power Control) andACM (Adaptive Coding and Modulation) capabilities in order to optimizesatellite resources utilization. The AUPC is designed to maintainconstant satellite transmitted power in all weather conditions bydynamically adapting the transmitted carrier level to the uplink rainattenuation. The ACM capability is designed to maintain constantreceived signal quality at each terminal by dynamically adapting themodulation and coding assigned to the packets transmitted to eachterminal to the downlink rain degradation affecting this terminal.

The following abbreviations are used throughout this specification:

AUPC—Automatic Uplink Power Control

ACM—Adaptive Coding and Modulation

CNR—Carrier to Noise Ratio

SIGL—Signal Level

NBW—Noise Bandwidth

HPA—High Power Amplifier

LNB—Low Noise Block

SCPC—Single Channel Per Carrier

PEB—Power Equivalent Bandwidth

In the following description and claims, the terms adapt and compensateare used synonymously. In general the concept of compensation is usedfor power whereas the concept of adaptation is used for modulation andencoding. In the present embodiments however, modification of the powerand of the modulation and encoding parameters are carried out in anintegrated fashion so that the two terms become synonymous.

The principles and operation of an apparatus and method according to thepresent invention may be better understood with reference to thedrawings and accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

Reference is now made to FIG. 1 which illustrates a controlled satellitelink, according to a first preferred embodiment of the presentinvention. A hub 10 transmits a signal to a satellite 12 over an uplink14. The uplink encounters rain and clouds 16 which cause weather-relatedattenuation of the signal. It will be appreciated that weatherconditions can change rapidly so that the overall attenuation of theuplink is itself liable to change rapidly.

The satellite 12 relays the signal it has received on the uplink to oneor more ground-based receiving stations 18 via a downlink 20. Thedownlink 20 is also liable to weather based attenuation, which may bebrought about by rain and clouds 22. It will be appreciated that thedynamic variation in attenuation on the downlink tends to add to anyattenuation on the uplink and also tends to vary independently. It isnoted that the uplink attenuation is present in all received signalssince there is only one uplink in the present embodiment, but thedownlink attenuation varies.

Thus in a first embodiment of the present invention a reference unit 24is inserted at a receiving station for measuring signal attenuation overthe link. The measured attenuation is transmitted back to the hub 10where a control unit 26, controls a link transmission parameter todynamically compensate for changes in the measured signal attenuation.Thus as the signal attenuation increases the reference unit 24 informsthe control unit, which then either strengthens the signal or makes thecoding or modulation or both more robust so that the received signalremains readable.

In FIG. 1, only a single ground-based receiving station is shown,although it will be appreciated that most satellites relay to multipleground stations. In fact the satellite link may be a broadcast link, andthere may therefore be numerous ground-based receiving stations spreadover a substantial region. In any event different weather conditions mayapply to different receiving stations.

Reference is now made to FIG. 2, which illustrates a further embodimentof the present invention in which the link of FIG. 1 is modified toprovide separate control over the uplink and the different down links.Parts shown in hashed lines may be regarded as theoretical since theability to make modifications to the satellite 12 is limited andpractical implementations are explained below. Specifically items shownin dashed lines indicate features which one would like to include at thesatellite, but in practice this is not possible and a system of indirectmeasurement is discussed below. Separate reference units are providedfor the uplink and all or some of the different down links. Referenceunit 28 is theoretically provided at the satellite for independentmeasuring of attenuation at the uplink, and control unit 26independently compensates for uplink attenuation. Reference unit 24measures attenuation on the downlink and control unit 30 at thesatellite independently compensates for changes in the measuredattenuation at the downlink. In practice reference unit 24 is all thatis available, so that uplink attenuation is derived from themeasurements at reference unit 24, as will be described in greaterdetail hereinbelow.

It is noted that in satellite communication there is a beacontransmitted at a different frequency with constant power towards theearth. Based on received beacon signal level the uplink attenuation canbe estimated after taking into account the frequency difference betweenthe beacon and the signal transmission.

In one embodiment a reference unit is provided at each receiving stationand the signal to each ground-based receiving station is independentlycontrolled. However, in the case of television or like broadcastingthere may be hundreds of thousands or even millions of receivingstations so, in an alternative embodiment, it is possible to aggregatethe various downlinks on a regional basis. That is all downlinks in acertain geographical area may be compensated together based on localweather as measured at one or two of the receiving stations in theregion. In an embodiment, only certain receiving stations, judiciouslydistributed, are used as references. Certain variations in attenuationmay be dealt with by attention to the uplink alone, whereas othervariations may require changes to the downlink. Stations that arereference stations may report all attenuation changes, but thosestations which are not reference stations need only report if thedownlink needs attention. More particularly, in order to reduce thenumber of interrupt signals, each individual VSAT can calculate thecurrent downlink attenuation or an indicator for zero downlinkattenuation and determine if variation that it measures in its CNRcorresponds also to downlink attenuation variation or only to uplinkattenuation variation. In the latter case a VSAT, which is not areference terminal, need not issue an interrupt with a request forMODCOD change but rather may simply wait for the AUPC to compensate forthe uplink attenuation variation.

Parameters used in transmission channels are numerous and many suchparameters can be adjusted to overcome attenuation. One such parameteris transmitted power. In case of severe attenuation the transmissionpower can be increased. Increased transmission power is generally onlyavailable from the hub 10 however. The satellite has only limited powerresources and thus increases in transmission power for the down link arenot really practical. Other parameters that can be modified are codingand modulation parameters. The complexity or robustness of the codingand/or modulation of the signal can be adjusted to maintain receivedsignal quality.

Reference is now made to FIG. 3, which is a simplified diagram showingan uplink 32 in which the controlled transmission parameter istransmission power. A downlink 34 is shown in which adaptive coding andmodulation are provided to ensure the quality of the received signal ismaintained. It will be appreciated that compensation for attenuation bymodifying the coding and modulation parameters to make the coding andmodulation more robust leads to a reduction in the signal rate. Thuspicture quality may have to be degraded, and say high definitiontelevision HDTV quality may be lost over the duration of a bad weatherepisode. However as long as the degradation is restricted to the badweather episode due to dynamic measuring of the signal then thedisruption to the customer is minimized. Alternatively, if the satellitebeam covers a large territory with many regions of independent climaticconditions, the throughput of a site can be maintained even in varyingrain conditions and accordingly varying modulation and codingparameters. The network design may take into account a distribution ofmodulation and coding parameters according to climate statistics overthe region. When a specific site uses more robust parameters it does nothave to reduce throughput but it can consume a larger fraction of thetotal carrier, while other sites may use less robust parameters at thesame time and therefore consume a smaller fraction of the carrier. For alarge network the actual aggregated throughput may be similar to thecalculated average throughput with very small variance.

Reference is now made to FIG. 4, which shows in greater detail how theinvention may be applied in practice to a broadcast type satellite linkwith a single hub and multiple receiving stations in which modificationsto the satellite are not possible. In FIG. 4 hub 10 broadcasts tosatellite 12 which relays the signal to ground-based receiving stations18.1 . . . 18.n. Each ground-based receiving station has differentweather conditions. The SNR at each receiving station is measured by ameasurement unit 24.1 . . . 24.n. The measurements are then fed viareturn links, which are typically satellite links or ground links 38,say ADSL over a telephone network, to AUPC and ACM controller 40. TheAUPC and ACM controller then interacts with ACM modulator 42 and boththe controller 40 and modulator 42 interact with traffic shaper 44 tomodify the signal that is sent over the link.

Two independent measurements of SNR and received signal level areperformed by a reference ground station or alternatively measurements offorward link and return link SNR of a reference ground station. The twomeasurements are considered together and enable estimations of theuplink and the downlink attenuation separately. Thus the inability tomeasure at the satellite is compensated for. The measurements fromdifferent ground stations are also considered together. Uplinkattenuation can be used to average the uplink result and downlinkattenuation is attributed to the different downlinks.

The present embodiments provide for coordination between the mechanismsthat compensate for uplink and downlink variations in the attenuation.Compensation for the uplink by changing the transmitted power affectsthe measurements performed by the ground station and the selection ofmodulation and coding parameters. Thus lack of coordination may resultin the repeating of transmissions of requests to change the currentselection from any of the ground stations before and after uplink powercompensation, so that the ground stations ask for a change that hasalready been provided. Furthermore the present embodiments require timefor achieving stable selection of parameters. The object of thecoordination is that different parts of the networks are not workingagainst each other and therefore preventing stability from beingattained. Failure to coordinate may lead to a need for increasedmargins, namely wasting satellite resources.

In summary there is provided a method of controlling a satellite linkcomprising: measuring attenuation over the link, and dynamicallyadjusting at least one of the transmission parameters to compensate forchanges in the measured attenuation. In an embodiment attenuation maytreated per leg, that is per uplink and per downlink, but in such acase, because the satellite itself cannot be modified, the effects ateach separate link have to be derived.

The presently derived approach may also be used for other ACM capableOutbound signals and also for point-to-point SCPC (Single Channel PerCarrier) satellite links. The embodiments use communication channelmeasurements, to allow location and beam independent, real timeoperation, of the combined AUPC and ACM processes. The channelmeasurements are used for estimating dependent or independent uplink anddownlink rain attenuation and degradation. These estimations are thenused for making the decisions on the compensations required in theuplink and in the downlink.

As will be explained in greater detail below, several principleimplementations are discussed. A first implementation, hereinafter CaseI, involves a reference terminal installed at the teleport. A secondimplementation, Case II involves reference terminals anywhere, namelyeither at the teleport or other locations in the same beam, or at otherlocations in a different beam. A third implementation, Case III involvesa return link via the satellite. This contrasts with FIG. 4 above, wherethe return link was terrestrial. In case III the return link providesmeasurements that are used together with forward link measurements forestimating the uplink and downlink attenuation. In case II the returnlink can be either via satellite or terrestrial and is used forforwarding the measurements made by the ground station relating to thelink from the ground station to the hub.

The present embodiments may be used for AUPC only, for example where ACMis not supported by terminals or not activated. Alternatively theembodiments may be used for ACM only, for example where a beaconreceiver is used for uplink power control, or uplink is transmitted viaC band beam, or the transponder operates at ALC—Automatic Level Controlmode. As a further alternative the embodiments may involve combined AUPCand ACM operating together to achieve optimal utilization of transponderresources.

The present embodiments provide a controller that compensates in realtime for independent atmospheric and other variations in both uplink anddownlink of a satellite communications link. Such a link may be eitherthe multiplexed Outbound carrier of a star VSAT network, or apoint-to-point SCPC satellite link. The compensation is performed forthe uplink by controlling the transmitted power in order to maintainconstant satellite transmitted power at all weather conditions. For thedownlink the compensation is based on assigning appropriate modulationconstellation and code rate which can provide the maximal throughput forthe actual weather conditions. The controller algorithm uses channelmeasurements performed by the receiving stations that are sent back tothe controller. The receiving stations are standard stations thatprovide service and can be anywhere, under any beam of the satellite.Measurements performed by several or all stations can be used forimproving the channel estimations. The uplink control is designed tomaintain constant satellite transmitted power at all weather conditionsby adapting the transmitted carrier level to the uplink rainattenuation. The adaptation of coding and modulation is designed tomaintain constant received signal quality at each terminal according tothe downlink rain degradation affecting this terminal. The adjustmentfor each terminal is implemented by the modulator by transmitting, usingtime-division multiplexing, a sequence of frames, where the coding andmodulation format may change frame-by-frame. Each frame may carrytraffic to terminals that expect the coding and modulation levelsassigned to that frame.

The uplink and down link adaptation are based on the same channelmeasurements. The present embodiments may separate the effects of theuplink and down link as reflected from the channel measurementsperformed by the receiving stations. As the uplink control influencesthe downlink performance, the present embodiments perform combinedcontrol of uplink and downlink by deducting the effect of the uplinkcontrol from the current channel measurements in order to allow forcomputing the downlink control stage using the same currentmeasurements. Such a technique reduces the control cycle time and thenumber of modulation and coding corrections as there is no need to waitfor the next updated measurements that would be affected by the uplinkupdate for correctly updating the downlink modulation and coding.

The above approach avoids repeating transmissions from all groundstations requesting to change selection of modulation and coding beforeand after uplink power modification, and saves time for achieving stableselection. Consequently smaller margins are required and satelliteresources are saved.

The channel estimations produced by the above process, namely uplink anddown link attenuations can be used, after appropriate correctionaccording to up/down frequency ratios, to additionally control thereturn links of a star VSAT network (or the return link of the SCPClink). The controller instructs each VSAT to increase/decrease its powerlevel in order to compensate for changes in the estimates of the Returnlink uplink attenuation. If the VSAT EIRP is already fully exploited andthe uplink rain-linked fading is not fully compensated, thencompensation may be achieved by a reduction in transmission rate and/ormodulation and coding, and the spare power may then be assigned to othermore powerful VSATs, so that the total power consumed from anytransponder is maintained at a constant level. The controller may alsoinstruct a modification of the transmission rate, modulation and codingin order to compensate the changes in downlink rain attenuation.Compensation may be based on either the already estimated downlink raindegradation or the measured return link signal to noise ratio.Compensation should be after deduction of the uplink power compensation.

Another consideration that may be taken into account by the controlleris to achieve balanced resource utilization, namely appropriateselection of modulation codes or MODCODs for the return links. That isto say the controller may wish to balance the consumed power andbandwidth resources from a transponder which contains the return linkswith or without the Outbound link. Balancing is based on having aselection of a few MODCODs for the return links where the higher MODCODsconsume more power equivalent bandwidth (PEB) than bandwidth, while thelower MODCODs consume more bandwidth than PEB. In the case that theOutbound link is in the same transponder it might be more efficient tomake the Outbound unbalanced, to allow a higher Outbound MODCOD, and tobalance it with appropriate selection of MODCODs for the return linksresiding in the same transponder. In the case of a band (or fulltransponder) assigned only for return links the controller may assignMODCODs according to traffic requirements, weather conditions, satellitecoverage, and balancing requirements so that overall balancing may beachieved. Such operation of the controller enables to use all availableresources in an efficient way. FIG. 6 is a graph showing the result ofsuch balancing. The controller takes into account that generally thereturn links are sensitive mainly to uplink fading and not to downlinkfading as the CNR in the downlink is generally much larger that in theuplink due to the use of a large teleport antenna. Therefore in thedesign of the balanced operation the assignment of MODCODs is mainlyaccording to overall network traffic in the return links that may bedelivered with specific MODCOD. The controller uses ACM and TRC(Transmission Rate Control) to compensate for the limitation of theremote terminal in terms of EIRP for severe rain conditions at theterminal site and for increasing the transmission rate beyond thecommitted rate to best effort based rates. Such a concept not onlyoptimizes the satellite transponder resources utilization but alsoallows minimizing of the required EIRP of the terminals and a reductionin their cost.

The concept is applicable to any form of modulation that the returnchannel may use. In particular it is applicable for both FDMA and TDMAtype return channels, where for TDMA the terminals may have to be movedamong carriers with different MODCODs or instantaneous transmission ratewhen the controller decides to change their MODCOD or theirinstantaneous transmission rate. In some TDMA implementations the MODCODcan be changed in the same carrier. The controller algorithm is asfollows for three active MODCODs, but can be extended to any number ofMODCODs:

-   -   1. Design in advance the ratios of overall network Inbound        traffic that may use High MODCOD, Medium MODCOD, and Low MODCOD        respectively to achieve balanced resource consumption. Balance        should be in terms of bandwidth and PEB in the frequency band        assigned to all the network carriers within given a transponder,        and may include the presence of an unbalanced Outbound carrier        in the same transponder, or may not.    -   2. Assign the High MODCOD to all terminals with low data rates        and good climate conditions until the maximum aggregated traffic        allowed to work in such MODCOD is reached, according to the        designed balance.    -   3. Assign Medium MODCOD to the rest of the terminals. These are        terminals which can survive at this time with the High MODCOD,        but are not far from exhausting their EIRP, add those terminals        where the CNR is such that they cannot use the High MODCOD and        thus could not otherwise deliver the actual required        transmission rate in their climate conditions. Add also those        terminals which can survive at this time with a higher MODCOD,        but are not far from exhausting their EIRP, until the designed        aggregated traffic for this MODCOD is reached.    -   4. Assign Low MODCOD to the rest of the terminals.    -   5. When traffic requirements or climate conditions change the        controller may change the assignments accordingly to maintain        the designed ratio for balanced resources consumption, while        taking into account the terminal limitations in terms of EIRP.    -   At the individual control stage the terminal tries to maintain        committed information rate (CIR) with its available EIRP        resources through power control, TRC (transmission Rate Control)        and ACM. When rain fading starts the VSAT initially requests        increasing its power. If all VSAT EIRP is exploited, and it is        above CIR, and still need more power to achieve the desired        Eb/No for its current MODCOD, it will reduce data rate down to        CIR, and then if further compensation is required it will reduce        the MODCOD. Power adjustment is performed per all changes in        data rate or MODCOD for achieving the desired Eb/No for such        MODCOD. Further compensation can be done by further reduction of        data rate below CIR, in order to maintain the link rather than        let it to break, thus increasing availability significantly        beyond the required availability, though with data rates below        CIR. Generally the design supports CIR for the required        availability with a certain MODCOD. When the rain fading        decreases, if the data rate is below CIR and it requests to        increase data rate, the controller will increase its data rate        (TRC) until it reaches CIR and if there is still available EIRP        to support a jump due to changing MODCOD, the MODCOD will be        increased for saving bandwidth. If there is still available        power the controller can either increase again the MODCOD or        allow increasing the data rate above CIR based on best effort.    -   After the individual stage where each VSAT adapts its power,        data rate, and MODCOD to the traffic requirements, and actual        rain conditions, the controller performs the system wise        adjustments of MODCOD assignments in order to achieve balanced        transponder resources utilization. At this stage VSATs that        could use high MODCOD may be switched to a lower one for the        purpose of balancing. Based on this higher level decision on        MODCOD the VSAT then adjusts it power accordingly to achieve the        desired Eb/No level. It might occur that an individual request        to change MODCOD is allowed by the controller, since this is the        only way to support CIR in requested availability, but then the        controller will command one or few other VSATs to change their        MODCODs in order to maintain balance.

We define the following parameters:

BW=Bandwidth in Hz

α=Roll Off Factor

COD=Code Rate of the FEC (Forward Error Correcting) Code

MOD=log₂( ) of modulation constellation size (e.g. 2 for QPSK, 3 for8PSK)

R_(s)=Symbol Rate in sps (symbols per second)

R_(b)=Bit Rate in bps (bits per second), R_(b)=R_(s)·(MOD·COD)

C=Carrier Power in Watts [(C) in dBW], after the receiver matched filter

N=Noise Power in Watts [(N) in dBW], after the receiver matched filter

NBW=Noise Bandwidth in Hz

N_(o)=Noise Spectral Density in Watts/Hz, i.e. Noise Power per 1 Hz,N_(o)=N/NBW

E_(s)=Energy per symbol in Joules, E_(s)=C/R_(s)

E_(b)=Energy per bit in Joules, E_(b)=C/R_(b)

CNR=Carrier to Noise Ratio [(CNR) in dB], CNR=C/N

SIGL=Received Signal Level in Watts [(SIGL) in dBW]

(G/T) (dB/K) is the figure of merit of a receiving terminal

L_(fs,dn) (dB) is the free space loss between the satellite and thereference VSAT at frequency f_(dn) (Hz)

A_(dn) (dB) is the downlink rain attenuation

A_(up) (dB) is the uplink rain attenuation

M_(CS) (dB) required clear sky margin

(CNR)_(HMC) (dB) the lower CNR threshold for the highest MODCOD (seeTable 3)

T_(antenna) (K) Antenna noise temperature

T_(LNB) (K) LNB noise temperature

HMC highest MODCOD allowed for clear sky conditions

AUPC Case I

We describe the algorithm for maintaining constant satellite transmittedpower EIRP_(sat) at all weather conditions by adapting the transmittedOutbound carrier level Tx_PWL to the uplink rain attenuation A_(up),where a reference VSAT, with antenna diameter D_(ref) is installed atthe teleport, and the same satellite beam covers both teleport and allother VSATs in the network, so that the carrier transmitted from theteleport may be received at the teleport. In Case I the referenceterminal is installed at the uplink teleport, thus having dependentuplink and downlink rain attenuation. CNR measurements and G/Tcorrections are used as proposed by Thomas J. Saam, “Uplink PowerControl Technique for VSAT Networks”, in Proceedings of Southeastcon 89,pp. 96-101, April 89, and Thomas J. Saam, “Uplink power controlmechanism for maintaining constant output power from satellitetransponder”, U.S. Pat. No. 4,941,199, Filed Apr. 6, 1989.

However the mechanism of deriving the uplink power control gain isdifferent as described in the following.

The received C/No can be expressed as follows:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right) = {{EIRP}_{sat} - L_{{fs},{dn}} - A_{dn} + \left( \frac{G}{T} \right)_{ref} - {k_{B}\left( {{dB}\; {Hz}} \right)}}} & (1.1)\end{matrix}$

where L_(fs,dn) (dB) is the free space loss between the satellite andthe reference VSAT at frequency f_(dn) (Hz) transmitted from thesatellite, A_(dn) (dB) is the downlink rain attenuation, (G/T)_(ref)(dB/K) is the figure of merit of the receiving reference terminal, andk_(B)=−228.6 dBW/HzK is the Boltzmann constant. The rain attenuation inthe uplink is related to the rain attenuation in the downlink asfollows:

A _(up) =K+A _(dn)(dB)  (1.2)

where for Ku band K is approximately 1.3 dB.

The relation between C/N_(o) and E_(b)/N_(o) is as follows:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right) = {\left( \frac{E_{b\;}}{N_{o}} \right) + {{10 \cdot {\log \left( {R_{s} \cdot {MOD} \cdot {COD}} \right)}}\left( {d\; {BHz}} \right)}}} & (1.3)\end{matrix}$

where R_(s) is the symbol rate, MOD is log₂( ) of the modulationconstellation size, and COD is the code rate.

In the following algorithm, the term ‘linkbudget’ refers to theaccounting of all of the gains and losses from the transmitter, throughthe medium (free space, cable, waveguide, fiber, etc.) to the receiverin a telecommunication system. It accounts for the attenuation of thetransmitted signal due to propagation, as well as the antenna gains,feedline and miscellaneous losses.

A simple link budget equation may be as follows:

Received Power (dBm)=Transmitted Power (dBm)+Gains (dB)−Losses (dB)

It is noted that decibels are logarithmic measurements, so addingdecibels is equivalent to multiplying the actual numeric ratios.

A more sophisticated listing of linkbudget components with exemplarymeasurements is given in table 1 below:

TABLE 1 A LinkBudget for a typical Satellite link. Tx BUC Rx Antenna BUCOBO Antenna Data Rate FEC Link Tx Location size (m) (Watt) (dB) size (m)(Kbps) Modulation FEC TYPE Outbound Best Teleport 9.10 400.00 18.16 1.2052000.00 16APSK 0.667 LDPC MODCOD Outbound Req. Teleport 9.10 400.0018.16 1.20 26000.00 QPSK 0.667 LDPC Aviblty Inbound R1A VSAT 1.20 2.003.54 9.10 32.00  8PSK 0.889 Turbo Best MODCOD Inbound R1B VSAT 1.20 2.007.34 9.10 32.00  8PSK 0.667 Turbo Req. Aviblty Space % Power % BW ofClear Sky Segment Rain Margin of trans- trans- Margin Link BER (KHz)Avilability (dB) ponder ponder (dB) Outbound Best 1.E−08 23400.00 91.00%2.03 71.68 65.00 2.46 MODCOD Outbound Req. 1.E−08 23400.00 99.70% 2.2371.68 65.00 Aviblty Inbound R1A 1.E−08 14.64 99.70% 2.00 0.08 0.04 BestMODCOD Inbound R1B 1.E−08 19.52 99.70% 2.00 0.03 0.05 Req. Aviblty

Algorithm steps for Case I.

-   -   (1) Determine from the linkbudget the highest MODCOD (denoted by        HMC) allowed for clear sky conditions so that a predefined        requirement for clear sky margin M_(CS) of e.g. 1 dB is met. The        required (C/N_(o)) for clear sky conditions (C/N₀)_(cs), at the        reference terminal, is calculated as follows:        (C/N₀)_(cs)=(CNR)_(HMC)+10·log(R_(S))+M_(CS)+M_(ref) (dB Hz),        where (CNR)HMC is the lower CNR threshold for the highest MODCOD        (see Table 3). If the diameter of the reference terminal is        different from the diameter of a typical VSAT antenna in this        network the difference M_(ref) in the clear sky margin obtained        should be compensated accordingly. This value can be obtained        from linkbudget tool by calculating the margin for the standard        antenna and for the reference antenna.    -   (2) Make calibration at clear sky conditions and determine the        Tx_PWL required to obtain the desired (C/No)_(cs). This is the        Tx_PWL_(cs) that obtains the desired EIRP_(sat) at clear sky        conditions. Calculate Tx_PWL_(max) by adding the uplink rain        fade as found by linkbudget tool for the desired uplink        availability. Measure the resulting (C/No)_(cs) for this        operating point and use the measured values in all calculations        rather than the linkbudget calculated value. This reduces        sensitivity to fixed measurement errors. (Note that calibration        can be in any MODCOD lower or equal to HMC).    -   (3) Measure (C/No) at predefined time intervals and perform        averaging over predefined number of measurements to obtain        (C/No)_(i) for the i-th interval. Search the solution for uplink        power control gain required at the i+1 time interval G_(upc,i+1)        satisfying the following expression

$G_{{upc},{i + 1}} = {\frac{1}{2} \cdot \left\{ {\left( \frac{C}{N_{o}} \right)_{cs} - \left( \frac{C}{N_{o}} \right)_{i + 1} + K + G_{{upc},i} - {10 \cdot {\log \left\lbrack {1 + {\left( \frac{T_{rain}}{T_{ref}} \right)\left( {1 - 10^{{- {({G_{{upc},{i + 1}} - K})}}/10}} \right)}} \right\rbrack}}} \right\}}$

Where typically T_(rain)=278 K, andT_(ref)=T_(antenna)/1.12+290*0.11+T_(LNB) (K) [6, pp. 191-192]. Thisexpression can be solved through numerical methods. It was found bysimulation that five iterations provide good accuracy. The iterationscan be started by substituting as initial guess G_(upc,i) inG_(upc,i+1), and generating through five iterations the G_(upc,i+1) forthe receiver quality (C/No)_(i+1).

-   -   (4) The new power level will then be

Tx _(—) PWL _(i+1) =Tx _(—) PWL _(cs) +G _(upc,i+1) (dBW)  (1.5)

A more detailed derivation is given hereinbelow.

AUPC Case II

In this Section we describe the algorithm for maintaining constantsatellite transmitted power EIRP_(sat) at all weather conditions byadapting the transmitted Outbound carrier level Tx_PWL to the uplinkrain attenuation A_(up), where a reference VSAT, with antenna diameterD_(ref) is installed either (1) at the teleport, and the same satellitebeam covers both teleport and all other VSATs in the network, or (2) atanother location, and the same satellite beam covers both teleport andall other VSATs in the network, or (3) at another location, anddifferent satellite beams cover the teleport and all other VSATs in thenetwork.

The solution for Case II is based on using measurements performed at thereference VSAT of both CNR (Carrier to Noise Ratio) and SIGL, thereceived signal level. The measurements can be reported either through areturn link or any other communication link.

The instantaneous CNR and SIGL can be read from the receiver chipsetwhich is typically composed of a tuner (e.g. STB6100) and a demodulator(e.g. STB0900).

Typically the CNR is measured after the received signal is filtered by asquare root raised-cosine matched filter with equivalent noise bandwidthNBW=R_(s), where R_(s) is the carrier symbol rate. Consequently

$\begin{matrix}{{N_{o} = {\frac{N}{NBW} = {\frac{N}{R_{s}}\left( {{Watts}\text{/}{Hz}} \right)}}}{and}} & (1.6) \\{{CNR} = {\frac{C}{N} = {\frac{E_{s}}{N_{o}} = {\frac{E_{b}}{N_{o}} \cdot {MOD} \cdot {{COD}.}}}}} & (1.7)\end{matrix}$

C/N_(o) can be expressed as a function of CNR and the symbol rate R_(s)

$\begin{matrix}{\frac{C}{N_{o}} = {{CNR} \cdot {{R_{s}({Hz})}.}}} & (1.8)\end{matrix}$

The received signal level SIGL is measured at the tuner IF input withbandwidth IFBW which is typically larger than the signal 3 dB bandwidth,Rs, in order to allow initial frequency error during acquisition stage.Consequently SIGL can be expressed as follows:

SIGL=C(1+β)+N(IFBW/Rs) (Watts)  (1.9)

where (1+β) is the ratio between signal power before and after thematched filter.

Equating N from (1.7) and (1.9) provides

$\begin{matrix}{{\frac{C}{CNR} = {\frac{{SIGL} - {C\left( {1 + \beta} \right)}}{{IFBW}/R_{s}}({Watts})}},} & (1.10)\end{matrix}$

resulting in the following expression for the carrier power C as afunction of the measured CNR and SIGL, the receiver filter bandwidthIFBW, and the matched filter factor (1+β)

$\begin{matrix}{C = {\frac{SIGL}{{\frac{IFBW}{R_{s}}\frac{1}{CNR}} + \left( {1 + \beta} \right)}{({Watts}).}}} & (1.11)\end{matrix}$

Algorithm steps for Case II

(1) Determine from the linkbudget the highest MODCOD (denoted by HMC)allowed for clear sky conditions so that a predefined requirement forclear sky margin M_(CS) of e.g. 1 dB is met. The required (C/N_(o)) forclear sky conditions (C/N₀)_(cs), at the reference terminal, iscalculated as follows:(C/N_(o))_(cs)=(CNR)_(HMC)+10·log(R_(S))+M_(CS)+M_(ref) (dB Hz), where(CNR)_(HMC) is the lower CNR threshold for the highest MODCOD (see Table3). If the diameter of the reference terminal is different from thediameter of a typical VSAT antenna in this network the differenceM_(ref) in the clear sky margin obtained should be compensatedaccordingly. This value can be obtained from linkbudget tool bycalculating the margin for the standard antenna and for the referenceantenna.

(2) Make calibration at clear sky conditions and determine the Tx_PWLrequired to obtain the desired (C/No)_(cs). This is the TX_PWL_(cs) thatobtains the desired EIRP_(sat) at clear sky conditions. CalculateTx_PWL_(max) by adding the uplink rain fade as found by linkbudget toolfor the desired uplink availability. Measure the resulting (C/No)_(cs)and (C)_(cs) for this operating point and use the measured values in allcalculations rather than the linkbudget calculated value. This reducessensitivity to fixed measurements errors. (Note that calibration can bein any MODCOD lower or equal to HMC).

(3) Measure (C/N_(o)) and (C) at predefined time intervals and performaveraging over predefined number of measurements to obtain (C/No)_(i+1)and (C)_(i+1). Solve the following expression for G_(upc,i+1), theuplink power control gain required at the i+1 iteration

$\begin{matrix}{G_{{ups},{i + 1}} = {G_{{upc},i} + \left\lbrack {C_{cs} - C_{i + 1}} \right\rbrack + {10{\log \left\lbrack {1 + {\frac{T_{{ref},{cs}}}{T_{rain}}\left( {1 - 10^{{\{{{\lbrack{{(\frac{C}{N_{o}})}_{cs} - {(\frac{C}{N_{o}})}_{i + 1}}\rbrack} - {\lbrack{C_{cs} - C_{i + 1}}\rbrack}}\}}/10}} \right)}} \right\rbrack}{({dB}).}}}} & (1.12)\end{matrix}$

Where typically T_(rain)=278 K, andT_(ref)=T_(antenna)/1.12+290*0.11+T_(LNB) (K). All other values in thisexpression are in dB.

The new transmitter power level will then be

Tx _(—) PWL _(i+1) =Tx _(—) PWL _(cs) +G _(upc,i+1) (dBW).  (1.13)

A detailed derivation is given hereinbelow.

(4) Optionally measurements can be performed by several referenceterminals, or by all terminals, for achieving more reliable decision forthe power control gain. Measurements that have large variance can befiltered out while the resulting power control gain per terminal fromthe other reference terminals can be averaged. Alternatively weightedaverage can be used where the weights are proportional to the CNR.Therefore after Polling, that is after requesting measurements from allreference terminals, a weighted average calculation may be performedwhere the weights are proportional to the CNR

$\begin{matrix}{{\overset{\_}{G}}_{{upc},{i + 1}} = {\frac{\sum\limits_{j}{G_{{upc},{i + 1},j}{CNR}_{j}}}{\sum\limits_{j}{CNR}_{j}}{({dB}).}}} & (1.14)\end{matrix}$

After Interrupt, that is after a terminal pushes its measurements whenit measures a significant change between Pollings, a weighted update ofthe last result may be performed

G _(upc,i+1)=(1−γ) G _(upc,i) +γG _(upc,i+1) (dB), 0<γ≦1  (1.15)

The last step of the algorithm is useful also for reducing the effect ofreference VSAT pointing loss. The algorithm cannot distinguish betweenrain and variations in pointing loss. Therefore, such variations inpointing loss of the teleport antenna or the reference terminals may beinterpreted erroneously as uplink rain attenuation as they do not affectthe VSAT noise level. The weighted average step can reduce the VSATpointing loss effect as the pointing loss varies independently from VSATto VSAT.

It is additionally noted that in order to reduce the number ofinterrupts, each individual VSAT can calculate the current downlinkattenuation or an indicator for zero downlink attenuation and todetermine if variation it measured in its CNR corresponds also todownlink attenuation variation or only to uplink attenuation variation.In the latter case a VSAT, which is not a reference terminal, will notissue an interrupt with a request for MODCOD change but will wait forthe AUPC to compensate for the uplink attenuation variation.

AUPC Case III

In this Section we describe the algorithm for maintaining constantsatellite transmitted power EIRP_(sat) at all weather conditions byadapting the transmitted Outbound carrier level TX_PWL to the uplinkrain attenuation A_(up), where a reference VSAT, with antenna diameterD_(ref) is installed either (1) at the teleport, and the same satellitebeam covers both teleport and all other VSATs in the network, or (2) atanother location, and the same satellite beam covers both teleport andall other VSATs in the network, or (3) at another location, anddifferent satellite beams cover the teleport and all other VSATs in thenetwork.

The solution for Case III is based on using measurements performed atboth ends of the link, e.g. at the Teleport and at the reference VSAT(or at both ends of SCPC link) of received CNR (Carrier to Noise Ratio)for both Forward and Return links. The measurements can be reportedeither through the return link or any other communication link.

The CNR equation that is shown by (B.14) can be used for both Forwardand Return links with appropriate indication of all parameters, where“F” stands for Forward link and “R” stands for Return link. For theForward link the expression is as follows:

$\begin{matrix}{{\,{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{i + 1}^{}}} = {{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{}^{}} + {{}_{}^{}{}_{{upc},i}^{}} - {{}_{}^{}{}_{{up},{i + 1}}^{}} - {{}_{}^{}{}_{{dn},{i + 1}}^{}} - {10{\log \left\lbrack {1 + {\frac{T_{rain}}{{}_{}^{}{}_{{ref},{cs}}^{}}\left( {1 - 10^{{- {{}_{}^{}{}_{{dn},{i + 1}}^{}}}/10}} \right)}} \right\rbrack}{\left( {d\; {BHz}} \right).}}}} & (2.16)\end{matrix}$

For the Return link the expression is as follows:

$\begin{matrix}{{\,{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{i + 1}^{}}} = {{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{}^{}} + {{}_{}^{}{}_{{upc},i}^{}} - {{}_{}^{}{}_{{up},{i + 1}}^{}} - {{}_{}^{}{}_{{dn},{i + 1}}^{}} - {10{\log \left\lbrack {1 + {\frac{T_{rain}}{{}_{}^{}{}_{{ref},{cs}}^{}}\left( {1 - 10^{{- {{}_{}^{}{}_{{dn},{i + 1}}^{}}}/10}} \right)}} \right\rbrack}{\left( {d\; {BHz}} \right).}}}} & (2.17)\end{matrix}$

The rain attenuation in the uplink is related to the rain attenuation inthe downlink, with a factor K_(T) for the teleport side and a factorK_(V) for the VSAT side, as follows:

_(F) A _(up) =K _(T)+_(R) A _(dn) (dB)  (2.18)

_(R) A _(up) =K _(V)+_(F) A _(dn) (dB)  (2.19)

Substituting for A_(dn) in the above CNR equations produces thefollowing two expressions for _(F)A_(up) and _(R)A_(up).

$\begin{matrix}{{{}_{}^{}{}_{{up},{i + 1}}^{}} = {K_{V} + {\left\{ {{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{}^{}} - {{}_{}^{}\left( \frac{C}{N_{o}} \right)_{i + 1}^{}} + {{}_{}^{}{}_{{upc},i}^{}} - {{}_{}^{}{}_{{up},{i + 1}}^{}} - {10{\log \left\lbrack {1 + {\frac{T_{rain}}{{}_{}^{}{}_{{ref},{cs}}^{}}\left( {1 - 10^{{- {({{{}_{}^{}{}_{{up},{i + 1}}^{}} - K_{V}})}}/10}} \right)}} \right\rbrack}}} \right\} \left( {d\; {BHz}} \right)}}} & (2.20) \\{{{}_{}^{}{}_{{up},{i + 1}}^{}} = {K_{T} + {\left\{ {{{}_{}^{}\left( \frac{C}{N_{o}} \right)_{}^{}} - {{}_{}^{}\left( \frac{C}{N_{o}} \right)_{i + 1}^{}} + {{}_{}^{}{}_{{upc},i}^{}} - {{}_{}^{}{}_{{up},{i + 1}}^{}} - {10{\log \left\lbrack {1 + {\frac{T_{rain}}{{}_{}^{}{}_{{ref},{cs}}^{}}\left( {1 - 10^{{- {({{{}_{}^{}{}_{{upj} + 1}^{}} - K_{T}})}}/10}} \right)}} \right\rbrack}}} \right\} \left( {d\; {BHz}} \right)}}} & (2.21)\end{matrix}$

These two equations can be solved with cross iterations, namelyinitially substituting guesses for both _(F)A_(up) and _(R)A_(up) in thefirst equation. N iterations are then performed for _(R)A_(up), and thenthe result is substituted in the second equation. Now N iterations areperformed for _(F)A_(up), and then the cross iterations are repeated Ntimes. Alternatively a look up table could be used.

Combined AUPC and ACM

The ACM mechanism can be operated to compensate for both uplink and downlink fades, or for downlink compensation independently, see Lawrence W.Krebs et al., “Methods and Apparatus For Mitigating Rain Fading OverSatcom Links Via Information Throughput Adaptation”, US PatentApplication Publication 2003/0054816, Filed Aug. 8, 2002; ETSI EN 302307 V1.1.1 (2004-01): “Digital Video Broadcasting (DVB) Secondgeneration framing structure, channel coding and modulation systems forBroadcasting, Interactive Services, News Gathering and other broadbandsatellite applications”; and Alberto Morello, Vittoria Mignone, “DVB-S2:The Second Generation Standard for Satellite Broad-band Services”,Proceedings of the IEEE, vol. 94, no. 1, pp. 210-227, January 2006. Inthe latter a beacon receiver is used for uplink power control, or uplinkis transmitted via a C band beam, or the transponder operates atALC—Automatic Level Control mode. The ACM mechanism can alternatively becombined with AUPC.

The present embodiments provide a combined AUPC and ACM controllerdesigned to achieve overall optimization based on allowed usage ofsatellite resources. The controller algorithm uses channel measurementsperformed by the receiving stations that are sent back to thecontroller. The receiving stations are standard stations that provideservice and can be located anywhere, under any beam of the satellite.Measurements performed by several or all stations can be used forimproving the uplink channel estimations. The uplink control is designedto maintain constant satellite transmitted power at all weatherconditions by adapting the transmitted carrier level to the uplink rainattenuation. The adaptation of coding and modulation is designed tomaintain constant received signal quality at each terminal according tothe downlink rain degradation affecting this terminal. The adjustmentfor each terminal is implemented by the modulator by transmitting, intime-division multiplex, a sequence of frames, where the coding andmodulation format may change frame-by-frame. The traffic of a terminalthat was assigned a specific MODOCD—see table 2 below, may betransmitted in the appropriate frame.

The uplink and down link adaptation are based on the same channelmeasurements. The present embodiments may separate the effects of theuplink and down link as reflected from the channel measurementsperformed by the receiving stations. As the uplink control influencesthe downlink performance, the present embodiments perform combinedcontrol of uplink and downlink by deducting the effect of the uplinkcontrol from the current channel measurements in order to allow forcomputing of the downlink control stage using the same current set ofmeasurements. This reduces the control cycle time and the number ofmodulation and coding corrections as there is no need to wait for thenext updated measurements that would be affected by the uplink updatefor correctly updating the downlink modulation and coding.

FIG. 4, already referred to above shows the scheme of an AUPC & ACMManagement system, comprising the AUPC & ACM Controller 40, the ACMmodulator 42, which includes the upconverter and the HPA—High PowerAmplifier, the Earth station 10, and the satellite 12. The satelliteterminals (VSAT) 18.1 . . . 18.n are connected to the AUPC & ACMController via return links. The terminals submit the CNR and SIGLmeasurements to the Controller. The ACM modulator operates at constantsymbol rate, since the available transponder bandwidth is assumed to beconstant. ACM is implemented by the modulator by transmitting, intime-division multiplex, a sequence of frames, where the coding andmodulation format may change frame-by-frame. Each frame can carrytraffic to terminals that know to expect the coding and modulationlevels assigned to that frame. Therefore, service continuity isachieved, during rain fades, by reducing user bits while increasing, atthe same time, the FEC redundancy and/or modulation ruggedness. Physicallayer adaptation is achieved as follows.

1) Each VSAT measures the channel status (CNR and SIGL) and reports itvia the return link to the Controller.

2) The VSAT reports are taken into account by the Controller fordeciding on updating the modulator Tx_PWL for compensating for uplinkdegradation and for selecting the MODCOD for data packets addressed tothat VSAT.

3) In order to avoid information overflow during fades, traffic shapingmay be implemented, using traffic shaper 44 to adapt the offered trafficto the available channel capacity. Thus for example during fades,television image quality may be degraded.

The AUPC and ACM update cycle is composed of the following stages:

1) Receiving updated channel status measurements,

2) Calculating uplink rain attenuation and updating the AUPC gaincontrol

3) Adding the increment in AUPC gain control to correct the channelmeasurements

4) Using the corrected channel measurements for selecting the MODCOD

The importance of making the correction phase within a combined AUPC andACM cycle is as follows: Both AUPC and ACM update can be performed onthe same set of channel measurements thus reducing the cycle period.Shortening the cycle period allows the required margin to be decreased.That is more efficient use is made of the scarce satellite resourcesallocated for compensating for fast rain fading. Otherwise if only AUPCis performed initially, ACM may be performed on a later measurement ofchannel status taken after the AUPC update already affected themeasurements.

The channel measurement correction can be expressed by

( CNR _(i+1))=(CNR _(i+1))+G _(upc,i+1) (dBHz).  (2.22)

See equations (2.7) and (2.8) above for the relations between(E_(b)/N_(o)) and (CNR) and between (C/N_(o)) and (CNR).

A typical table with selection of MODCODs for DVB-S2 is shown as Table 2below. A typical example for a MODCOD threshold table showing the upperand lower thresholds for selecting a MODCOD is given in Table 3 below.The (CNR) ranges for neighbor MODCODs are partly superposed in order toreduce number of MODCOD switching when (CNR) is near the border betweentwo MODCODs. The combined process of AUPC and ACM is shown in the flowchart of FIG. 5. Periodic polling is carried out of all VSATs (receivingstations). On periodical Polling of all VSATs. Interrupts are generatedby individual VSATs and occur between Polling events when the particularVSAT needs to correct its MODCOD for maintaining its received signalquality. In order to reduce the number of interrupts, each individualVSAT can calculate the current downlink attenuation based on expression(B.16) and determine if the variation it has measured in its CNRcorresponds also to downlink attenuation variation or only to uplinkattenuation variation. In the latter case a VSAT, which is not areference terminal, will not issue an interrupt with a request forMODCOD change but will wait for the AUPC to compensate for the uplinkattenuation variation. We can thus define the following expression

$\left\{ {\left\lbrack {\left( \frac{C}{N_{o}} \right)_{cs} - \left( \frac{C}{N_{o}} \right)_{i + 1}} \right\rbrack - \left\lbrack {\left( C_{cs} \right) - \left( C_{i + 1} \right)} \right\rbrack} \right\}$

dB as an indicator for downlink attenuation as if it equals zero (orclose to zero with predefined accuracy) the downlink attenuation inexpression (B.16) is also zero.

Mode MODCOD QPSK 1/4  1_(D) QPSK 1/3  2_(D) QPSK 2/5  3_(D) QPSK 1/2 4_(D) QPSK 3/5  5_(D) QPSK 2/3  6_(D) QPSK 3/4  7_(D) QPSK 4/5  8_(D)QPSK 5/6  9_(D) QPSK 8/9 10_(D) QPSK 9/10 11_(D) 8PSK 3/5 12_(D) 8PSK2/3 13_(D) 8PSK 3/4 14_(D) 8PSK 5/6 15_(D) 8PSK 8/9 16_(D) 8PSK 9/1017_(D) 16APSK 2/3 18_(D) 16APSK 3/4 19_(D) 16APSK 4/5 20_(D) 16APSK 5/621_(D) 16APSK 8/9 22_(D) 16APSK 9/10 23_(D) 32APSK 3/4 24_(D) 32APSK 4/525_(D) 32APSK 5/6 26_(D) 32APSK 8/9 27_(D) 32APSK 9/10 28_(D) Reserved29_(D) Reserved 30_(D) Reserved 31_(D) DUMMY  0_(D) PLFRAME MODCOD Table2 from ETSI EN302307 reference above.

TABLE 3 Example for MODCOD Thresholds Table CNR Spetral Ideal CNR LowerUpper Ideal Efficiency Recommended Allowed Lists MODCOD Modulation MODCode Rate CNR Threshold Threshold Bo/No bps/Hz Pllots QPSK Q/8PSK Q/8/16Q/8/16/32 1 QPSK 2 0.250 −2.36 −infinity −0.44 0.7 0.42 Off v v v v 2QPSK 2 0.333 −1.24 −0.64 0.50 0.5 0.56 Off v v v v 3 QPSK 2 0.400 −0.300.30 1.80 0.7 0.67 Off v v v v 4 QPSK 2 0.500 1.00 1.60 2.80 1.0 0.83Off v v v v 5 QPSK 2 0.600 2.23 2.60 3.50 1.4 1.00 Off v v v v 6 QPSK 20.667 3.10 3.30 4.40 1.9 1.11 Off v v v v 7 QPSK 2 0.750 4.03 4.20 5.002.3 1.25 Off v v v v 8 QPSK 2 0.800 4.68 4.80 5.60 2.6 1.33 Off v v v v9 QPSK 2 0.833 5.18 5.40 6.50 3.0 1.39 Off v v v v 10 QPSK 2 0.889 6.206.40 6.90 3.7 1.48 Off v 11 QPSK 2 0.900 6.42 6.70 +infinity 3.9 1.50Off v 12  8PSK 3 0.600 5.50 6.00 7.10 2.9 1.50 On v v v 13  8PSK 3 0.6676.62 6.90 8.40 3.6 1.67 On v v v 14  8PSK 3 0.750 7.91 8.20 9.80 4.41.88 On v v v 15  8PSK 3 0.833 9.35 9.70 11.30 5.4 2.08 On v 16  8PSK 30.889 10.69 11.10 11.60 6.4 2.22 On v 17  8PSK 3 0.900 1.98 11.40+infinity 6.7 2.25 On v 18 16APSK 4 0.667 8.97 9.47 10.91 4.7 2.22 On vv 19 16APSK 4 0.750 10.21 10.71 11.73 5.4 2.50 On v v 20 16APSK 4 0.80011.03 11.53 12.31 6.0 2.67 On v v 21 16APSK 4 0.833 11.61 12.11 13.506.4 2.78 On v v 22 16APSK 4 0.889 12.89 13.39 13.83 7.4 2.96 On v 2316APSK 4 0.900 13.13 13.63 +infinity 7.6 3.00 On v 24 32APSK 5 0.75012.73 13.23 14.34 7.0 3.13 On v 25 32APSK 5 0.800 13.64 14.14 14.98 7.63.33 On v 26 32APSK 5 0.833 14.28 14.78 16.39 8.1 3.47 On v 27 32APSK 50.889 15.69 16.19 16.75 9.2 3.70 On v 28 32APSK 5 0.900 16.05 16.56+infinity 9.5 3.75 On v Note if the lower threshold is crossed goingdownward, the MODCOD will be reduced If the upper threshold is crossedgoing upward, the MODCOD will be increased

In other words the combined process of AUPC and ACM, as shown in FIG. 5is based on periodical Polling of all VSATs and obtaining interruptsgenerated by individual VSATs between Polling events when the VSAT needsto correct its MODCOD for maintaining its received signal quality.

Selection of MODCODs for analysis and efficient Operation

Reference is now made to FIG. 7, which is a simplified diagramillustrating MODCOD and bandwidth relationships. For the purpose ofanalysis and efficient operation it is desirable to simplify thescenario. We propose here two stages of reducing the number ofoperational MODCODs.

Stage 1: Partition the service territory into regions characterized bysignificantly different satellite coverage strength and/or climateconditions. Select two MODCODs per each such region by assuming twomodes of operation, Mode 1: “Highest MODCOD” (HMC) which can be used inthe region based on the satellite EIRP and earth stations capabilities,for near to clear sky conditions, and the availability that correspondsto such a MODCOD, called “Derived Availability” (A_(HMC)). Typically theavailability that reflects near to clear sky conditions will be about95%. Mode 2: “Required Availability” (A_(RQ)) and the corresponding“Derived MODCOD” (DMC) that can satisfy such availability. Such twomodes with appropriate MODCODs prevail in each region.

-   -   We can use efficiency in terms of bps/Hz (bps stands for bit per        second) as an indication of the achieved throughput or consumed        bandwidth per each MODCOD. The efficiency per MODCOD is give by

g=MOD*COD/(1+α)

The total efficiency per region is defined by

g _(i) =g _(HMC) ·A _(HMC) +g _(DMC)·(A _(RQ) −A _(HMC))

The system efficiency G is calculated using the traffic distribution asfollows:

$G = \frac{\sum\limits_{i}{{Traffic}_{i} \cdot g_{i}}}{\sum\limits_{i}{Traffic}_{i}}$

-   -   Where Traffic_(i) is the aggregate traffic for region i. The        following table 4 describes a case study based on the above        method. For example in Region 2, HMC is 16APSK 0.833 and is        active A_(HMC)=96.0% of the time, and DMC is 8PSK 0.75 and is        active A_(RQ)−A_(HMC)=99.7%−96.0%=3.7% of the time, achieving as        result the required availability of 99.7%.

TABLE 4 Case study for analysis based on partitioning to regions and twoMODCODs per region. Region 1 Region 2 Region 3 Region 4 Best MODCODThroughput, Mbps 105,000 100,000 80,000 105,000 MOD 16APSK 16APSK 16APSK16APSK COD 0.875 0.833 0.667 0.875 Availability % A_(HMC) 99.30 96.0094.00 98.45 Efficiency bps/Hz 2.92 2.78 2.22 2.92 Req. AvailabilityThroughput, Mbps 90,000 67,500 36,000 80,000 MOD 16APSK 8PSK QPSK 16APSKCOD 0.750 0.750 0.600 0.667 Availability % A_(RQ) 99.70 99.70 99.7099.70 Delta Availability % 0.40 3.70 5.70 1.25 Efficiency bps/Hz 2.501.88 1.00 2.22 Total Outbound Total efficiency 2.91 2.74 2.15 2.90performance bps/Hz Traffic Distribution % 52.7 4.2 37.1 6.0 Systemefficiency 2.62 bps/Hz ACM gain 162%

For the purpose of operation it is also desirable to reduce the numberof instantaneously operational MODCODs. The ACM based carrier (e.g.DVB-S2) is built from blocks of coded traffic. Each block has a fixedMODCOD for the traffic carried in it. The traffic that waits fortransmission in the buffer is waiting for a block with the appropriateMODCOD. If the number of MODCODs is large there are many queues oftraffic waiting for a turn to be transmitted. Traffic with a rarely usedMODCOD may indeed have to wait a long time until their turn comes. Therewill be large variations in the delay which are not suitable forinteractive applications. For the above case study, FIG. 8 describes thedistribution of MODCODs. This distribution is generated by weighing eachMODCOD with its activity factor (availability for HMC or Deltaavailability for DMC) and with the traffic fraction using it, namely thetraffic per that region scaled by the total traffic. Actually we canreduce at this stage the number of MODCODs to those selected in theanalysis described above and achieve the performance obtained by theanalysis. In the case study shown here six different MODCODs are needed.

Stage 2: Further reduction of the number of MODCODs in order toeliminate MODCODs with low utilization.

A method for further reduction in the number of MODCODs can be based onusing the set of MODCODs selected in Stage 1 and eliminating those oflow utilization, e.g. less than 1% of the time. The rule is that trafficthat needs a certain MODCOD may fall to the next low allowed MODCOD. Insuch a method the lowest MODCOD should be kept in the allowed list. Inthe case study shown here two of the six MODCODs that remained afterStage 1 may be eliminated with insignificant degradation in the systemefficiency. FIG. 9 illustrates a series of MODCODs each with differentlevels of traffic.

Rules for adjusting the MODCOD table (Table 3):

-   -   When few of the MODCODs are disabled, the thresholds will be        calculated as follows:    -   1. The lower Threshold of the lowest allowed MODCOD is unlimited        (−infinity).    -   2. The lower thresholds of allowed MODCODs (other than the        lowest allowed MODCOD) are in force.    -   3. The upper thresholds of allowed MODCODs are recalculated:

Upper_Threshold(Any_MODCOD)=Lower_Threshold(NEXT_higher_allowedMODCOD)+Margin,

-   -   where the margin is typically 0.2 dB.    -   4. The upper threshold of the highest allowed MODCOD is        unlimited (+infinity)

In the following we provide the detailed derivation of the expressionfor the uplink power control gain required at the i+1 iterationG_(upc,i+1) for case 1 above.

The received C/N_(o) can be expressed as follows:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right) = {{EIRP}_{sat} - L_{{fs},{dn}} - A_{dn} + \left( \frac{G}{T} \right)_{ref} - {k_{B}\mspace{14mu} \left( {{dB}\; {Hz}} \right)}}} & \left( {A{.1}} \right)\end{matrix}$

At the i+1 iteration the transmitted EIRP becomes:

EIRP _(i+1) =EIRP _(sat) −A _(up,i+1) +G _(upc,i) (dBW)  (A.2)

Where EIRP_(sat) is the EIRP that should be maintained constant,A_(up,i+1) is the rain attenuation at the i+1 iteration, and G_(upc,i)is the control gain applied at the i-th iteration. Consequently thereceived C/N_(o) will become:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {{EIRP}_{i + 1} - L_{{fs},{dn}} - A_{{dn},{i + 1}} + \left( \frac{G}{T} \right)_{{ref},{i + 1}} - k_{B}}} & \left( {A{.3}} \right)\end{matrix}$

Substituting (A.1) at clear sky (A_(dn)=0) into (A.2) and the result in(A.3), and also using the relation A_(up)=K+A_(dn), the followingexpression is obtained

$\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} + L_{{fs},{dn}} - \left( \frac{G}{T} \right)_{{ref},{cs}} + k_{B} - A_{{up},{i + 1}} + G_{{upc},i} - L_{{fs},{dn}} - A_{{up},{i + 1}} + K + \left( \frac{G}{T} \right)_{{ref},{i + 1}} - k_{B}}$

After simplification it becomes

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} - \left( \frac{G}{T} \right)_{{ref},{cs}} - {2A_{{up},{i + 1}}} + K + G_{{upc},i} + {\left( \frac{G}{T} \right)_{{ref},{i + 1}}\left( {{dB}\; {Hz}} \right)}}} & \left( {A{.4}} \right)\end{matrix}$

Consequently the estimated uplink rain attenuation can be expressed as

$\begin{matrix}{A_{{up},{i + 1}} = {\frac{1}{2}\begin{Bmatrix}{\left( \frac{C}{N_{o}} \right)_{cs} - \left( \frac{C}{N_{o}} \right)_{i + 1} + K +} \\{G_{{upc},i} - \left\lbrack {\left( \frac{G}{T} \right)_{{ref},{cs}} - \left( \frac{G}{T} \right)_{{ref},{i + 1}}} \right\rbrack}\end{Bmatrix}({dB})}} & \left( {A{.5}} \right) \\{G_{{upc},i} = {{Tx\_ PWL}_{i} - {{Tx\_ PWL}_{cs}\mspace{11mu} ({dB})}}} & \left( {A{.6}} \right)\end{matrix}$

Finally the control gain for the i+1 iteration should be found from thefollowing expression

$\begin{matrix}{G_{{upc},{i + 1}} = {{\frac{1}{2} \cdot \begin{Bmatrix}{\left( \frac{c}{N_{o}} \right) - \left( \frac{C}{N_{o}} \right)_{i + 1} + K + G_{{upc},i} - {10 \cdot}} \\{\log\left\lbrack {1 + {\left( \frac{T_{rain}}{T_{ref}} \right)\left( {1 - 10^{{- {({G_{{upc},{i + 1}} - K})}}/10}} \right)}} \right\rbrack}\end{Bmatrix}}({dB})}} & \left( {A{.7}} \right)\end{matrix}$

Where typically T_(rain)=278 K, andT_(ref)=T_(antenna)/1.12+290*0.11+T_(LNB) (K) See Maral and Bousquet pp.191-192.

In the following we provide the detailed derivation of the expressionfor the uplink power control gain required at the i+1 iteration, usingmeasurement of CNR and Signal Level, for CASE II above.

Development of the Received Carrier Power equation:

The received carrier power can be expressed by

(C)=EIRP _(sat) −L _(fs,dn) −A _(dn) +G _(ref) −A _(Rx) (dBW)  (B.1)

where L_(fs,dn) (dB) is the free space loss between the satellite andthe reference VSAT at frequency f_(dn) (Hz) transmitted from thesatellite, A_(dn) (dB) is the downlink rain attenuation, G_(ref) (dB) isthe gain of the reference terminal antenna, and A_(Rx) is the receiverRF/IF chain loss. It is assumed that the CNR at the uplink is high andall the EIRP_(sat) transmitted by the satellite is used only by thedesired signal.

At the i+1 iteration the transmitted EIRP becomes

EIRP _(i,+1) =EIRP _(sat) −A _(up,i+1) +G _(upc,i) (dBW).  (B.2)

Where EIRP_(sat) is the EIRP that should be maintained constant,A_(up,i+1) is the rain attenuation at the i+1 iteration, and G_(upc,i)is the control gain applied at the i-th iteration. Consequently thereceived carrier power (C) will become:

(C _(i+1))=EIRP _(i+1) −L _(fs,dn) −A _(dn,i+1) +G _(ref) −A _(rx)(dBW).  (B.3)

Substituting (B.1) at clear sky (A_(dn)=0) into (B.2) and the result in(B.3), the following expression is obtained:

(C _(i+1))=(C _(cs))+L _(fs,dn) −G _(ref) +A _(Rx) −A _(up,i+1) +G_(upc,i) −L _(fs,dn) −A _(dn,i+1) +G _(ref) −A _(Rx) (dBW)  (B.4)

where (C_(cs)) is the received carrier power at clear sky. Aftersimplification it becomes the Carrier Power equation:

(C _(i+1))=(C _(cs))+G _(upc,i) −A _(up,i+1) −A _(dn,i+1) (dBW).  (B.5)

Development of the CNR Equation:

The received C/N_(o) can be expressed as follows:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right) = {{EIRP}_{sat} - L_{fsdn} - A_{dn} + \left( \frac{G}{T} \right)_{ref} - {{k_{B}\left( {{dB}\; {Hz}} \right)}.}}} & \left( {B{.6}} \right)\end{matrix}$

At the i+1 iteration the transmitted EIRP becomes:

EIRP _(i+1) =EIRP _(sat) −A _(up,i+1) +G _(upc,i) (dBW).  (B.7)

Where EIRP_(sat) is the satellite EIRP that should be maintainedconstant, A_(up,i+1) is the rain attenuation at the i+1 iteration, andG_(upc,i) is the control gain applied at the i-th iteration.Consequently the received (C/N_(o)) will become:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {{EIRP}_{i + 1} - L_{{fs},{dn}} - A_{{dn},{i + 1}} + \left( \frac{G}{T} \right)_{{ref},{i + 1}} - {{k_{B}\left( {{dB}\; {Hz}} \right)}.}}} & \left( {B{.8}} \right)\end{matrix}$

Substituting (B.6) at clear sky (A_(dn)=0) into (B.7) and the result in(B.8), the following expression is obtained:

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} + L_{{fs},{dn}} - \left( \frac{G}{T} \right)_{{ref},{cs}} + k_{B} - A_{{up},{i + 1}} + G_{{upc},i} - L_{{fs},{dn}} - A_{{dn},{i + 1}} + \left( \frac{G}{T} \right)_{{ref},{i + 1}} - {k_{B}\left( {{dB}\; {Hz}} \right)}}} & \left( {B{.9}} \right)\end{matrix}$

After simplification it becomes

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} + G_{{upc},i} - A_{{up},{i + 1}} - A_{{dn},{i + 1}} - {\left\lbrack {\left( \frac{G}{T} \right)_{{ref},{cs}} - \left( \frac{G}{T} \right)_{{ref},{i + 1}}} \right\rbrack {\left( {{dB}\; {Hz}} \right).}}}} & \left( {B{.10}} \right)\end{matrix}$

As per Maral and Bousquet page 31, the difference in received noisetemperature can be expressed by:

ΔT=T _(ref,i+1) −T _(ref,cs) =T _(rain)(1−10^(−A) ^(dn,i+1) ^(/10))(K).  (B.11)

The difference in the figure of merit G/T can be expressed by

$\begin{matrix}\begin{matrix}{\left\lbrack {\left( \frac{G}{T} \right)_{{ref},{cs}} - \left( \frac{G}{T} \right)_{{ref},{i + 1}}} \right\rbrack = {10{\log \left( \frac{T_{{ref},{i + 1}}}{T_{{ref},{cs}}} \right)}}} \\{= {10{\log \left( {1 + \frac{\Delta \; T}{T_{{ref},{cs}}}} \right)}{({dB}).}}}\end{matrix} & \left( {B{.12}} \right)\end{matrix}$

By substituting a ΔT from equation (B.11) the following expression isobtained

$\begin{matrix}{\begin{bmatrix}{\left( \frac{G}{T} \right)_{{ref},{cs}} -} \\\left( \frac{G}{T} \right)_{{ref},{i + 1}}\end{bmatrix} = {10{\log\left\lbrack {1 + {\frac{T_{rain}}{T_{{ref},{cs}}}\left( {1 - 10^{{- A_{{dn},{i + 1}}}/10}} \right)}} \right\rbrack}{({dB}).}}} & \left( {B{.13}} \right)\end{matrix}$

Finally the CNR equation is obtained by substituting (B.13) into (B.10):

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} + G_{{upc},i} - A_{{up},{i + 1}} - A_{{dn},{i + 1}} - {10{\log\left\lbrack {1 + {\frac{T_{rain}}{T_{{ref},{cs}}}\left( {1 - 10^{{- A_{{dn},{i + 1}}}/10}} \right)}} \right\rbrack}{\left( {{dB}\; {Hz}} \right).}}}} & \left( {B{.14}} \right)\end{matrix}$

Now by combining the Carrier Power Equation (B.5) and the CNR Equation(B.14) through equating G_(upc,i)−A_(up,i+1)−A_(dn,i+1) the followingexpression is obtained

$\begin{matrix}{\left( \frac{C}{N_{o}} \right)_{i + 1} = {\left( \frac{C}{N_{o}} \right)_{cs} + \left( C_{i + 1} \right) - \left( C_{cs} \right) - {10{\log\left\lbrack {1 + {\frac{T_{rain}}{T_{{ref},{cs}}}\left( {1 - 10^{{- A_{{dn},{i + 1}}}/10}} \right)}} \right\rbrack}\left( {{dB}\; {Hz}} \right)}}} & \left( {B{.15}} \right)\end{matrix}$

Which after simplification leads to the following expression for thedown link attenuation

$\begin{matrix}{A_{{dn},{i + 1}} = {{- 10}{\log\left\lbrack {1 + {\frac{T_{{ref},{cs}}}{T_{rain}}\left( {1 - 10^{{\{{{\lbrack{{(\frac{C}{N_{o}})}_{cs} - {(\frac{C}{N_{o}})}_{i + 1}}\rbrack} - {\lbrack{{(C_{cs})} - {(C_{i + 1})}}\rbrack}}\}}/10}} \right)}} \right\rbrack}{({dB}).}}} & \left( {B{.16}} \right)\end{matrix}$

Consequently by using the Carrier Power Equation (B.5) the uplink rainattenuation can be expressed by

A _(up,i+1)=(C _(cs))−(C _(i+1))+G _(upc,i) −A _(dn,i+1) (dB)  (B.17)

where the gain control G_(upc,i) at the i-th iteration can be expressedby the transmitter power level at the i-th iteration with respect topower level at clear sky

G _(upc,i) =Tx _(—) PWL _(i) −Tx _(—) PWL _(cs) (dB).  (B.18)

Finally the control gain applied at the i+1 iteration G_(upc,i+1) shouldbe equal to A_(up,i+1) in order to maintain EIPR_(sat) constant asrequired

G _(upc,i+1) =A _(up,i+1)=(C _(cs))−(C _(i+1))+G _(upc,i) −A _(dn,i+1)(dB)  (B.19)

where A_(dn,i+1) is given by (B.16).

It is expected that during the life of this patent many relevant devicesand systems will be developed and the scope of the terms herein, isintended to include all such new technologies a priori.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents, and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. A controlled satellite link comprising a first link and a secondlink, comprising: at least one reference unit for obtaining at leastsignal attenuation over said controlled satellite link, and a controlunit, associated with said reference unit, configured for discriminatingbetween first link attenuation and second link attenuation andaccordingly controlling at least one link transmission parameterassociated with said first link and at least one transmission parameterassociated with said second link to dynamically adapt to changes in saidobtained signal attenuation.
 2. A controlled satellite link according toclaim 1, comprising said first link and a plurality of second links andwherein said reference unit comprises respective reference units locatedat least some of said second links, and wherein said satellite linkfurther comprises a derivation unit for deducting common attenuation andassigning said common attenuation at least in part to said first linkand assigning differential attenuation to respective second links,thereby to adapt independently to changes in said measured attenuationat each respective second link.
 3. A controlled satellite link accordingto claim 1, wherein said controlled transmission parameter comprises anyone of the group consisting of transmission power, a coding parameter, amodulation parameter, the combination of transmission power in saidfirst link and a coding parameter in said second link, the combinationof transmission power in said first link and a modulation parameter insaid second link, the combination of transmission power in said firstlink and both coding and modulation parameters in said second link. 4.The apparatus of claim 1, wherein said satellite link has a first endand a second end and said attenuation is obtained through measurementsboth at said first end and at said second end.
 5. A controlled satellitelink according to claim 2, wherein said independent reference units aredistributed on a geographical basis to allow independent control of saidplurality of links on said geographical basis.
 6. A controlled satellitelink according to claim 2, wherein measurements by said reference unitare processed in a plurality of stages and said derivation unit isconfigured to obtain said first link attenuation in a first stage andsaid second link attenuation in a second stage.
 7. A controlledsatellite link according to claim 1, wherein said controlledtransmission parameter is transmission power in said first link and acombination of coding and modulation parameters in said second link,said link further configured to optimize a total of said combinationsoperational within said satellite link at any given time.
 8. Acontrolled satellite link according to claim 1, wherein said control isin order to balance between consumption of satellite resources.
 9. Acontrolled satellite link according to claim 8, wherein said balancingbetween consumption of satellite resources comprises optimizing betweenbandwidth and power output.
 10. A method of controlling a satellitelink, said satellite link transmitting according to transmissionparameters, the method comprising: measuring attenuation over said link,said link having two legs, a first leg to a satellite and a second legto respective ones of a plurality of ground based receiver stations,said measuring being carried out at some of said ground based receiverstations, each leg being associated with respective ones of saidtransmission parameters, said measuring discriminating betweenattenuation at said respective legs; and dynamically adjusting at leastone of said transmission parameters associated with said first leg andat least one of said transmission parameters associated with said secondleg to adapt for changes in said discriminated measured attenuation. 11.The method of claim 10, wherein the at least one transmission parameteradjusted is transmission power and said dynamically adjusting comprisesincreasing said transmission power at said first leg to adapt forincreases in said measured attenuation and decreasing said transmissionpower to adapt for decreases in said measured attenuation, and adjustingmodulation levels for said second leg such that a signal quality afterdecoding of the received signal at said plurality of ground basedreceiver stations is kept substantially constant.
 12. The method ofclaim 10, wherein the at least one transmission parameter adjusted is acoding parameter, and said dynamically adjusting comprises increasingcoding complexity to compensate for increases in said measuredattenuation, and decreasing coding complexity to compensate fordecreases in said measured attenuation such that the signal qualityafter decoding of the received signal is kept substantially constant.13. The method of claim 10, wherein the at least one transmissionparameter adjusted is a coding parameter, and said dynamically adjustingcomprises decreasing code rate to adapt for increases in said measuredattenuation, and increasing code rate to adapt to decreases in saidmeasured attenuation such that the signal quality after decoding of thereceived signal is kept substantially constant.
 14. The method of claim10, wherein the at least one transmission parameter adjusted is amodulation parameter, and said dynamically adjusting comprisesdecreasing modulation constellation size to adapt to increases in saidmeasured attenuation, and increasing modulation constellation size toadapt to decreases in said measured attenuation such that the signalquality after decoding of the received signal is kept substantiallyconstant.
 15. The method of claim 10, wherein said at least oneparameter comprises one member of the group consisting of: a)transmission power for said first link and a coding or modulationparameter for said second link; and b) transmission power for said firstlink and a coding parameter and a modulation parameter for said secondlink.
 16. The method of claim 10, wherein said first link is an uplinkfrom a ground-based transmitting station to a satellite, and said secondlink is a downlink from said satellite to a ground-based receivingstation.
 17. The method of claim 16, wherein said satellite link is abroadcasting link, and there are a plurality of ground-based receivingstations.
 18. The method of claim 17, wherein at least some of saidground-based receiving systems carry out said measuring, and commonattenuation is assigned to the uplink and differential attenuation isassigned to respective downlinks.
 19. A method of compensating fordynamic changes in attenuation in a satellite link having a first linkand a second link and attenuation over said first link and attenuationover said second link, comprising: measuring said dynamic changes inattenuation at a receiving station of said satellite link,discriminating using said measurements between dynamic changes relatingto attenuation at said first link and dynamic changes relating tochanges at said second link; feeding said measurements via a returnlink; and using said measurements and said discrimination, modifying atleast one transmission parameter associated with said first link and onetransmission parameter associated with said second link to adapt to saiddynamic changes.
 20. A satellite receiving station for receiving aforward link channel of a satellite communication link; comprising: ameasuring unit for obtaining measurements of dynamic changes inattenuation of said channel, said measurement including data to allowdiscrimination between attenuation in a first leg and attenuation in asecond leg of said satellite link; and an output unit for sending saidmeasurements via a return channel to allow for channel compensation. 21.The satellite receiving station of claim 20, wherein said output unit isconfigured to send measurements relating to attenuation in said forwardlink.